The present invention relates to intervertebral disc replacements and, in particular, it concerns an intervertebral disc replacement which allows motion about three rotational axes and provides motion attenuation to avoid end motion impact in motion about each of the axes.
Many commonly used Total Disc Replacements (TDR) are based upon a ball-and-socket type articulated joint. While a ball-and-socket is effective to provide a full range of motion in all directions, it does not provide motion attenuation. Specifically, with regard to axial rotation, the ball-and-socket joint itself inherently allows effectively unlimited rotation with minimal resistance. The vertebral section therefore relies upon surrounding soft tissue or impact of the facets to limit the motion, thus accelerating degeneration of the facets and the adjacent tissue. In anterior flexion, posterior extension and lateral bending, the motion is typically limited by impact of edges of the TDR flange upon each other, generating repeated impacts during normal use which may lead to degeneration of the TDR itself, or of the surrounding tissue.
More specifically, and to provide a reference for comparison with the present invention to be described below, FIGS. 11A-11C show cross-sectional views of a TDR based on ball-and-socket geometry in various positions of lateral bending, while FIGS. 11D and 11E show the bending moment and the elevation (corresponding to “intervertebral spacing”, to be defined below) as a function of angular displacement from a central (“neutral”) position. The normal range of motion of the joint is from the neutral position of FIG. 11A to the flange-on-flange contact position of FIG. 11B, here corresponding to a deflection of about 6 degrees. Within this range, as shown in FIG. 11D, minimal bending moment is required to generate motion, only needing to overcome any frictional resistance of the joint. Once contact occurs between the flanges, there is a sudden resistance to further deflection such that a gradually increasing applied moment does not generate any further motion. Only if the torque exceeds a certain threshold, deflection will continue by lifting the ball out of the socket as shown in FIG. 11C, a state which is not normally intended to occur. Once started, the lifting motion will continue at constant applied torque. The corresponding graph of intervertebral spacing as a function of angular deflection is shown in FIG. 11E. Within the normal ball-and-socket operating range of 0-6 degrees, no elevation occurs. If sufficient torque is then applied to start lifting the ball out of the socket, the intervertebral spacing starts to increase steeply, initially as an approximately linear function of deflection angle.
Various attempts have been made to develop an articulation arrangement for a TDR which more closely emulates various aspects of the dynamics of the natural intervertebral disc. These include a wide range of devices employing cylindrical bearings or saddle-like articulation surfaces. Examples of such devices may be found in the following U.S. Pat. Nos. 6,706,068; 6,908,484; 6,972,037; 6,972,038; 6,986,789; 6,989,032; 6,994,727; 6,994,729; 6,997,955; 7,048,764; and 7,048,766, and in the following US Patent Application Publication Nos.: 2004/0225364; and 2004/0073311.
Of particular interest as background to the present invention, some of the above-referenced documents introduce a concept of distraction (i.e., increased intervertebral spacing) as a function of angular displacement of axial rotation in order to generate restorative (self-centering) forces. Specifically, since the normal state of the spinal column is to be loaded axially with the weight of the upper body, an articulation arrangement which causes distraction of the joint as a function of axial rotation performs work against the loading, resulting in a restoring force which tends to return the joint to a neutral position of axial rotation. An example of such teachings may be found in the aforementioned U.S. Pat. No. 6,994,727.
Although the concept of joint distraction under applied load to provide a restoring force is discussed in the aforementioned document, it is only applied in a single mode of motion, namely, axial rotation. Furthermore, the documents currently known to the inventor do not provide any acceptable solution for motion attenuation in order to limit the range of motion for each type of motion without causing impact or otherwise endangering surrounding tissue.
There is therefore a need for an intervertebral disc replacement which would allow motion about three rotational axes and provide motion attenuation to avoid end motion impact in motion about each of the axes.